1. Field of Invention
This invention relates to noise measurements and more particularly to an automated noise measurement system for wireless transmitters.
2. Related Art
In general, the purpose of an electrical, electro-optic, and electro-acoustic communication system is to transmit information-bearing signals (generally known as “baseband signals”) through a communication channel separating a transmitter from a receiver. The term baseband signal (also known as baseband wave, baseband waveform, or just a “baseband”) is used to designate the band of frequencies representing the original information-carrying signal as delivered by a source of information.
The efficient utilization of the communication channel typically requires a shift of the range of baseband signal's frequencies into other frequency ranges suitable for transmission through the communication channel, and then a corresponding shift back to the original frequency range after reception. As an example, a typical radio system generally must operate with signals having frequencies of 30 kHz and above, whereas the baseband signal usually contains frequencies in the audio frequency range (i.e., 20 to 20 kHz frequency range), and so some form of frequency-band shifting must be used for the radio system to operate satisfactorily. A shift of the range of frequencies in a signal may be accomplished by using the process of “modulation” that is defined as the process of varying some characteristic of a carrier signal (also known as a carrier wave, carrier waveform, or just a “carrier”) in accordance with a modulating signal (also known as a modulating wave or modulating waveform). The carrier signal is typically a signal having a continuous waveform (which is usually electrical) whose properties are capable of being modulated, or impressed, with a second signal that is an information-carrying signal (i.e., the modulating signal). Generally, the carrier signal itself conveys no information until altered in some fashion, such as having its amplitude changed, its frequency changed, or its phase changed by the modulating signal. These modulating signal induced changes convey information via the resulting signal from the modulation process. In this situation, the baseband signal is referred to as the modulating signal, and the result of the modulation process is referred to as a modulated signal (or modulated wave). At the receiving end of the communication system, it is usually required that the modulating signal (i.e., the original baseband signal that is also the original information-carrying signal) be restored so as to receive the information carried on the modulating signal. This is accomplished by using a process known as demodulation, which is the reverse of the modulation process.
Unfortunately, noise in electrical, electro-optic, and electro-acoustic systems may disrupt both the amplitude and/or phase of signals that are utilized and/or processed by these types of systems. In this situation, the term “noise” is used to designate unwanted signals that tend to disturb the transmission and processing of the desired signals in electrical, electro-optic, and electro-acoustic systems. However, because many of these types of systems are relatively insensitive to fluctuations in amplitude, the fluctuations in phase (denoted as “phase-noise”) are generally more problematic. As an example, an oscillator is a device capable of producing an oscillating output signal (such as, for example, a signal having a sinusoidal waveform generally known as a “sinusoid signal”) having an oscillating frequency. Generally, an oscillator includes some type of amplitude-limiting feature that reduces the effects of any potential fluctuations in amplitude such that phase-noise is typically the major noise contributor to the resulting oscillator output signal.
Because noise, such as phase-noise, is such an important factor in the design and/or performance of electrical, electro-optic, and/or electro-acoustic systems, designers typically desire a measure of the phase-noise for a given system. In the past, various approaches have been utilized to characterize the phase-noise of a given system. For example, amplifiers have been characterized by first injecting an input signal of a known frequency into an input of an amplifier and then measuring the resulting amplified output signal with a spectrum analyzer where the spectrum analyzer is a device capable of displaying the spectral waveform composition of some electrical, acoustic, or optical signals including the respective amplitude intensity and frequency of a given signal. Unfortunately, the measurement sensitivity of this approach is limited by the relatively poor sensitivity of the spectrum analyzer. Moreover, it is difficult to measure the phase-noise at frequencies values that are close to the carrier frequency of the signal, where the carrier frequency is the frequency of the carrier signal.
Unlike a spectrum analyzer, a phase-locked discriminator system has relatively good sensitivity and generally allows measurements close to the carrier frequency. However, configuring a phase-locked discriminator system is cumbersome and time consuming. In attempt to solve this problem, an Automated Phase-locked Discriminator Noise-test Measurement System (“APD System”) was developed as described in U.S. Pat. No. 6,793,372 (which is herein incorporated by reference in its entirety) that attempts to alleviate the cumbersome nature of such systems. In FIG. 1, a functional block diagram is shown of an example of an implementation of an APD System 100 in signal communication with a unit-under-test (“UUT”) 102 and spectrum analyzer 104 via signal paths 106, 108, and 110, respectively.
The APD System 100 includes a variable low-noise source 112, variable phase-shifter 114, variable amplifier 116, mixer 118, variable low-noise matching amplifier 120, analog-to-digital converter (“ADC”) 122, and controller 124. The UUT 102 is in signal communication with both the variable low-noise source 112 and variable amplifier 116 via signal paths 106 and 108, respectively. The mixer 118 is in signal communication with the variable phase-shifter 114, variable amplifier 116, and variable low-noise matching amplifier 120 via signal paths 126, 128, and 130, respectively. The variable phase-shifter 114 is also in signal communication with the variable low-noise source 112 via signal path 132. The ADC 122 is in signal communication with both the variable low-noise matching amplifier 120 and controller 124 via signal paths 134 and 136, respectively. The controller 124 is in signal communication with the spectrum analyzer 104, variable low-noise source 112, variable phase-shifter 114, variable amplifier 116, and variable low-noise matching amplifier 120 via signal paths 110, 138, 140, 142, and 144, respectively.
As an example of operation, the variable low-noise source 112 produces a UUT input signal 146 (which is low-noise carrier signal) for driving the UUT 102. The UUT 102 may be any device for which a user desires a phase-noise test measurement such as, for example, an amplifier, phase-shifter, diplexer or other suitable device or system of devices. The UUT 102 receives the UUT input signal 146, via signal path 106, and processes it to produce a UUT output signal 148. As an example, if UUT 102 is an amplifier, the UUT output signal 148 would be an amplified version of the UUT input signal 146. The UUT output signal 148 is received, via signal path 108, and amplified by the variable amplifier 116 to produce a variable amplifier signal 150 that is passed to mixer 118 via signal path 128. The variable low-noise source 112 also produces a variable phase-shifter input signal 152 that is passed to the variable phase-shifter 114 via signal path 132. The variable phase-shifter input signal 152 is identical to the UUT input signal 146 and has the same frequency as the UUT input signal 146. The variable phase-shifter 114 phase shifts the variable phase-shifter input signal 152 by 90 degrees to produce a variable phase-shifted signal 154 that is passed to the mixer 118 via signal path 126. In this fashion, the carrier signal of the variable amplifier signal 150 is eliminated from a mixer output signal 156 produced by the mixer 118. To keep the mixer output signal 156 in the proper dynamic range of the ADC 122, the mixer output signal 156 is passed, via signal path 130, to and processed by the variable low-noise matching amplifier 120 to produce a variable low-noise matched output signal 158 that is passed to the ADC 122 via signal path 134.
To eliminate the carrier signal of the variable amplifier signal 150, the variable phase-shifted signal 154 must be in quadrature (i.e., shifted 90 degrees) with respect to the carrier signal. If a quadrature relationship between the variable amplifier signal 150 and variable phase-shifted signal 154 is not established, a DC offset will be present in a digital ADC output signal 160 from the ADC 122.
The controller 124 monitors the ADC output signal 160 and controls the variable phase-shifter 114 using a variable phase-shifter control signal 162 to maintain the quadrature relationship between the variable amplifier signal 150 and variable phase-shifted signal 154. The variable phase-shifter control signal 162 is sent to the variable phase-shifter 114 via signal path 140.
The elimination of the carrier signal from the variable amplifier signal 150 also depends upon whether the carrier signal (i.e., UUT input signal 146) and the phase-shifted version of the carrier (i.e., variable phase-shifted signal 154) are of equal power when entering the mixer 118. Thus, analogous to the control of the variable phase-shifter 114, the controller 124 also controls the variable amplifier 116 responsive to processing the ADC output signal 160 using a variable amplifier control signal 164, via signal path 142, to maintain equal powers for the variable phase-shifted signal 154 and variable amplifier signal 150. These powers need not be maintained exactly equal but instead may merely be within a sufficient range of each other so that linear operation of the mixer 118 is assured. Those of ordinary skill in the art will appreciate that variable amplifier 116 does not just amplify but may also attenuate responsive to the variable amplifier control signal 164. As an example, if the UUT 102 is an amplifier, the variable amplifier 116 may have to attenuate the UUT output signal 148 to keep both the variable phase-shifted signal 154 and variable amplifier signal 150 in comparative power equality. The controller 124 may also control the variable low-noise matching amplifier 120 utilizing a variable low-noise matched amplifier control signal 166, via signal path 144, to maintain the variable low-noise matched output signal 158 in a proper dynamic range for the ADC 122.
Having controlled the components for quadrature operation, the controller 124 eliminates the carrier signal from the ADC output signal 160 such that the ADC output signal 160 simply represents the phase-noise. The phase-noise injected by the variable low-noise source 112 may be accounted for by a calibrating operation such that the UUT 102 is removed and the variable low-noise source 112 simply feeds the variable amplifier 116 directly, although such a direct feed may occur through a delay line (not shown). The resulting phase-noise in the ADC output signal 160 during calibration maybe stored in a memory (not shown) associated with controller 124. Thus, during testing of the UUT 102, the controller 124 (or the spectrum analyzer 104 associated with the controller 124) may perform a Fourier analysis of the ADC output signal 160 to determine the phase-noise power. The measured phase-noise may then be adjusted by the phase-noise injected by the variable low-noise source 112 to determine the additive phase-noise supplied by the UUT 102.
The phase-noise measured in the ADC output signal 160 depends upon the frequency of the UUT input signal 146. For example, the UUT 102 may be quite noisy at one frequency but less so at another. To measure phase-noise across a range of frequencies, the controller 124 may command the variable low-noise source 112 to change the frequency of the UUT input signal 146 utilizing a variable low-noise source command signal 168, via signal path 138, measure the resulting phase-noise, change the frequency again, measure the resulting phase-noise after the change of frequency, and so on. Advantageously, such measurement is performed automatically and accurately with no manual intervention or tailing as would be necessary in conventional phase-noise test measurement systems.
Unfortunately, although the APD System 100 represents a significant advance in the art, certain challenges still remain because many factors are involved in properly biasing or driving a given component for optimum low-noise performance. As an example, if the UUT 102 is a fiber-optic link, the APD System 100 will not be able to test the fiber-optic link without extensive manual involvement from a user because the fiber-optic link will typically require manual biasing for optimal performance so that it may be properly tested by the APD System 100.
As an example, in FIG. 2 a functional block diagram of a conventional fiber-optic link 200 is shown. The fiber-optic link 200 may include an input amplifier 202, laser diode 204, fiber-optic channel 206, photo-detector 208, and output amplifier 210. In this example, the laser diode 204 may be in signal communication with the input amplifier 202 and fiber-optic channel 206 via signal paths 212 and 214, respectively. The photo detector 208 may be in signal communication with the fiber-optic channel 206 and output amplifier 210 via signal paths 216 and 218, respectively.
In an example of operation, the input amplifier 202 amplifies an electrical input signal sin(t) 220 and produces a laser input signal 222 to drive the laser diode 204. In turn, the laser diode 204 drives an input optical signal 224 into the fiber-optic channel 206 (that may be an optical fiber). After passing through fiber-optic channel 206, the output optical signal 226 is converted into an electrical signal 228 in the photo-detector 208. The output amplifier 210 then amplifies the electrical signal 228 to provide an output signal sout(t) 230. Unfortunately, many factors are involved in properly biasing the fiber-optic link 200 for optimal performance with an APD System. For example, matching input and output amplifiers 202 and 210 to the fiber-optic link 200 that includes properly biasing the transistors within input and output amplifiers 202 and 210 and properly biasing the laser diode 204 and photo-detector 208 are all factors that affect the additive phase-noise that the fiber-optic link 200 injects into the output signal sout(t) 230. However, a designer of the fiber-optic link 200 has no intelligent way of setting these factors. A similar situation exists for the proper setting of variables in many other systems and devices.
Additionally, although the APD System 100 may be advantageously used to characterize the phase-noise performance of many different types of UUTs 102, the APD System 100, unfortunately, requires a non-wireless signal path to the variable low-noise source 112 such as, for example, a transmission line that may include a coaxial cable or a coplanar waveguide to carry the source signal. There are transmitters such as those utilized in cellular telephones in which it is very inconvenient or impossible to access the source signal through such a signal path.
Accordingly, there is a need in the art for automated systems that are capable of measuring the phase-noise performance of wireless transmitters through reception and analysis of a transmitted wireless signal.